Spinal canal stenosis is to be understood as any form of narrowing of the spinal canal through which the spinal chord runs, whilst excluding inflammation, tumors and complete slipped disks. Most commonly, spinal canal stenosis takes place in the region of the lumbar spine, but it also occurs in the region of the thoracic spine and cervical spine. The most common cause of the acquired spinal canal stenosis is degenerative changes on the spinal column. Such degenerative changes of the bone-cartilage system occur more commonly with older patients. Therapeutically, one initially intervenes with anesthetic medication, such as non-steroid anti-rheumatics or by way of creating a lordosis correction by way of bandages or corsets. If this is inadequate, then one must intervene in an operative manner, for example with decompressive surgery. Since, as already mentioned, the stenosis patients are often older people and they often also suffer from co-morbid conditions, the risk of complications is considerable. Accordingly, one is interested in being able to perform minimal-invasive surgery which reduces the risk of complication.
In place of the very widespread stiffening of several vertebral bodies in the lumbar region, in recent years elements have been developed which may be inserted between two adjacent vertebra processes and are designed to be adjustable in height. Such implants are shown, for example, in U.S. Pat. No. 5,458,641 or U.S. Pat. No. 5,702,455. The use of these implants however requires the operator to largely open up the lumbar spinal region, in order to insert the respective element and in particular to carry out the required size adaptation. This problem has been recognized, and a corresponding solution has been suggested, which has a simplified implant which may be adapted in size in a self-regulating manner by an elastic intermediate element. The advantage of this solution is not only simpler adaptation to the bodily particularities, but also the simple construction of the element and its fixation on the vertebral processes. It would also be conceivable to apply such a solution using merely a unilateral access. However, one would not do this for safety reasons, since the element with the processes is screwed, and as a result one would operate on both sides of the spinal column for a secure control.
Further implants for dealing with lumbar spinal canal stenosis are known from EP-0,322,334 A and FR-2,724,554 A. The solution described in WO99/21501 is particularly advantageous. The implant disclosed therein operates with a central body on which a sleeve is rotatably mounted in an eccentric manner. As locking means, wings are provided on both sides of this sleeve which is oval in cross section, and these wings must bear on the central body on both sides of the vertebral processes. This size adaptation by way of the spacer which is oval in cross section, requires more space to be provided for this, and furthermore the design is set up such that the operator must have access to both sides of the vertebral column.
The main advantage of this solution is that one requires no type of screwing on the vertebral body, and the implant itself is relatively simple in its construction. There are differing opinions with regard to the requirements of an element which may be adapted in diameter to the respective situation. Certain operators are of the opinion that the size of the spacer to be inserted may be exactly defined by way of an exact diagnostic clarification and measurement. Thereby, one wishes to assume the smallest required diameter of the spacer. The smaller the implant and the smaller the operation, the lower the risk of post-operative complications.